Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply coatings to various objects to improve or change the properties of the objects. For example, plasma deposition processes may be used to apply wear resistant coatings to machine tools, while other types of coatings may be used to increase the corrosion resistance of other items, such as bearings, turbine blades, etc, thereby enhancing their performance. In still other applications, plasma deposition may be used to apply coatings to various types of surfaces in the optics and glass industries.
In most plasma deposition processes the plasma is created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field. The electric field, which is typically created between two electrodes, ionizes the process gas and creates the plasma. In the case of a DC sputter deposition plasma process, the material (commonly referred to as the target) to be deposited on the object or substrate is connected as the negative electrode (i.e., cathode), whereas some other element, commonly the vacuum chamber itself, is connected as the positive electrode or anode. Ionized process gas atoms comprising the plasma are accelerated toward and ultimately impact the negatively charged cathode/target, dislodging or sputtering atoms from the target material. The sputtered atoms subsequently condense on various items in the chamber, including the substrate that is to be coated. The substrate is usually positioned with respect to the target so that a majority of the sputtered target atoms condense on the surface of the substrate.
Sputter deposition processes of the type described above are usually referred to as "non-reactive" processes in that the sputtered atoms deposited on the surface of the substrate do not react with other materials. However, sputter deposition processes have also been developed wherein the target material is sputtered in the presence of a reactive material (e.g., oxygen or nitrogen gas). Such "reactive" sputtering processes may be used to deposit a film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition processes.
While it is a relatively simple matter to connect a power supply (e.g., a DC power supply) to the electrodes and generate the plasma, it is considerably more difficult to produce and control the plasma so that the deposition process will consistently produce coatings of the desired quality. One of the main difficulties in this regard is that the plasma is generally quite unstable and is subject to rapid and significant variations in electrical impedance. These impedance variations are related to a variety of factors, including the type of material being deposited, the pressure within the chamber, the strength of the magnetic field used to confine the plasma, and other extrinsic factors. Regardless of the cause, the impedance variations create many problems and contribute to certain phenomenon that occur within the chamber, most of which are deleterious to the deposition process.
One common phenomenon is the periodic occurrence of electrical discharges or arcs within the vacuum chamber. Such electrical discharges or arcs can take on different forms depending on the characteristics of the apparatus and on the particular plasma process being used. For example, arcs may occur between the target material and the substrate itself, certainly causing a defect in the coating, if not ruining the substrate entirely. Alternatively, arcs may occur between the target and some other part of the vacuum chamber, in which case the deleterious effects of such arcs are usually less severe, but nevertheless tend to degrade the overall quality of the coating. The arcs can be short lived, lasting only a few milliseconds or so, or may be continuous, again depending on the particular apparatus and process being used.
The impedance variations and plasma phenomenon (e.g., arcing) described above generally preclude the use of a simple DC power supply in most applications. As a result, many different types of power supplies have been developed over the years in an effort to compensate for these problems. For example, most power supplies used to accomplish DC sputter deposition processes may be operated in either a constant current or constant power mode. Operating the power supply so that it provides constant current or power to the plasma tends to minimize some of the problems relating to the impedance instability of the plasma and enhances the overall deposition process. Unfortunately, such power supplies do little to reduce the occurrence of arcs, and systems utilizing such power supplies still tend to be plagued by the occurrence of arcs within the process chamber.
Partly in an effort to reduce the occurrence of arcs within the process chamber, many DC power supplies have been developed which are capable of temporarily removing the voltage potential on the electrodes when an arc condition is detected. One such type of power supply is the phase controlled SCR power supply. Essentially, this type of power supply utilizes one or more silicon controlled rectifiers (SCRs) to turn-off the power supply when desired (e.g., such as upon the detection of an arc condition within the chamber), then turn it back on when the condition subsides. While most phase controlled SCR power supplies can be turned off in the time required for the AC input current to cross through zero volts (i.e., in about 8.33 milliseconds (ms) for a single phase 60 Hz input), such a response time is almost always too slow to be of any use, particularly for arc suppression purposes. While SCR power supplies utilizing three-phase input power can be turned on and off in approximately one-third the time (e.g., in about 2.8 ms for a three-phase 60 Hz input), this is still not fast enough for satisfactory arc suppression in most plasma processes.
Another type of power supply design is the switching power supply. In a switching power supply, the AC input power is first rectified to DC. The DC is then converted back to AC by a suitable switching converter. Such switching type power supplies are capable of considerably faster switching speeds (i.e., response times) compared with phase controlled SCR power supplies, thus are generally preferred over the SCR power supplies.
Unfortunately, however, switching power supplies have not proven to be a panacea and many are plagued with their own problems and disadvantages. For example, while most switching power supplies are theoretically capable of much faster response times than phase controlled SCR supplies, their response times are often limited by the numerous feedback networks often required for the stable, controlled operation of the power supplies, particularly when the power supplies are operated at low power levels. Indeed, when operated at low power levels, the feedback networks of many power supplies reduce the effective response times of the supplies to the point where they perform little better than the SCR supplies which they were designed to replace. Yet another problem with switching power supplies is that they are prone to excessive power dissipation, which usually limits their maximum power output. Consequently, switching power supplies often cannot be used in plasma processes requiring high input powers. Still another problem with switching power supplies is that many supplies contain excessive capacitance in the switching devices which, again, tends to limit the response times of the power supplies and also tends to create instabilities when they are operated at low power levels.
Accordingly, a need remains for an improved DC power supply for plasma processing systems. Such an improved power supply should be capable of fast response to allow maximum control over the plasma, particularly when operated at low power levels. Additional advantages could be realized if the power supply could also be operated over a wide range of plasma impedances and with a minimum of internal power dissipation.